With the fast development of nanomaterials and nanotechnology, environmental nanotechnology has attracted increasing concerns in the past decades. In the field of water treatment, nanotechnology exhibited great potential in improving the performance and efficiency of water decontamination as well as providing a sustainable approach to secure water supply. In this review, the current applications of nanomaterials in water and wastewater treatment were briefly discussed. The synthesis and physiochemical properties of diverse free nanomaterials, including carbon based nanomaterial, metal and metal oxides nanoparticles as well as noble metal nanoparticles, were focused on, and their performance and mechanisms towards removal of various contaminants were discussed. When concerning the large-scale application in water treatment, nanoparticles have to face some inherent technical bottlenecks such as aggregation, difficult separation, leakage into the contact water, as well as potential adverse effect imposed on ecosystem and human health. The emerging nanocomposite materials integrate the merits of functional nanoparticles and varying solid hosts of large size, and exhibit great advantages in scaled-up application. This review particularly covered the topic of environmental nanocomposites, such as those of organic and inorganic supports, nanocomposite membranes and magnetic nanocomposites. The advantages and perspectives of various nanocomposites are briefly discussed.